The present invention relates to three-dimensional display systems and more particularly to such a system employing a reflective plate weighted at its rim and vibrating in an essentially resonant mode.
Three-dimensional (3-D) display systems have been proposed previously in which successive image components, generated in a single plane, are caused to appear juxtaposed in space when viewed in a membrane mirror which is vibrated to effect a varying focal length. Such mirrors have been constructed as diaphragms which yield useful curvatures owing to the tension in the membrane which opposes a pressure differential across the diaphragm produced, for example, by a loudspeaker. One such system is described in U.S. Pat. No. 3,493,290, issued Feb. 3, 1970 to Alan C. Traub and also in a May 1968 Mitre Corporation report M68-4 entitled "A New 3-Dimensional Display Technique".
Diaphragm mirrors, however, have several inherent problems. In order to obtain a flicker-free operation, it is necessary to vibrate the mirror at a frequency of about 30 cycles per second. In that such frequencies are within or adjacent the audible or sensory range, the size of the diaphragm becomes limited by the acoustic energy given off by the mirror. In other words, the near field sound pressure level adjacent a large vibrating diaphragm may be simply intolerable for a person who should be in close proximity to the mirror in order to carefully view the image. Likewise, as a diaphragmatic mirror is made larger, its natural resonant frequency goes down. Thus, any attempt to oscillate it at a flicker-free frequency may cause the excitation of higher modes of oscillation which are acoustically undesirable and which will create severe distortions in the appearance of the image.
Another problem with diaphragmatic mirrors is that the mirror displacement and shape are highly sensitive to the spectral purity of the driving waveform and any distortion in the signal energizing the loudspeaker or any distortion generated in the loudspeaker itself or any distortions generated in the air coupling between loudspeaker and diaphragmatic mirror can distort the desired juxtaposition in space of the image elements. Accordingly, it has typically been impossible to utilize the travel of the diaphragm in both directions for the reason that the velocity profile in the one direction is not the exact complement of the velocity profile in the other direction. While the departure from sinusoidal movement does not distort unacceptably an image produced during travel in one direction, the effect of having a displacement between image components which are intended to appear in the same plane, though generated during alternate half-cycles, is defeating of the entire purpose of the display. A further problem is that such diaphragms, being typically constructed as metalized plastic films, are very difficult to clean without damage.
Among the several objects of the present invention may be noted the provision of a display system providing three-dimensional images of high quality; the provision of such a display system which will create images of relatively large size; the provision of such a display system which involves relatively low acoustic output; the provision of such a display system which is relatively long-lived and easy to maintain; the provision of such a display system which is highly reliable and of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.